The present invention relates to a method of producing a semiconductor device.
Conventionally, in order to form an element isolation area in the production of a semiconductor device, the LOCOS (Local Oxidation of Silicon) method has been used. As shown in FIGS. 4(a) to 4(c), the LOCOS method is a technique of laminating an oxidized insulating film (SiO.sub.2) 51 and a silicon nitride film 52 on a silicon (Si) substrate 50 (FIG. 4(a)), and selectively oxidizing silicon using as a mask the patterned silicon nitride film 53 to form a thick silicon oxide film 54 (FIGS. 4(b) and 4(c)). In this method, as shown in FIG. 4(c), the silicon oxide film 54 extends long and thin from the boundary of the element isolation area to an active area 55 for formation of an element (the extended area is referred to as a bird's beak). This bird's beak narrows the active area. Thus, in order to prevent the bird's beak from being generated, an improved LOCOS method has been proposed in which polycrystalline silicon (poly-Si) is used as a buffer film as shown in FIGS. 5(a) to 5(c) (see Japanese Patent Unexamined Publication No. Hei 2-260639). In this improved method, a poly-Si film 62 is formed on a silicon oxide film 61 on a Si substrate 60, and a silicon nitride film 63 is patterned on the poly-Si film 62 (FIGS. 5(a) and 5(b)). Using the silicon nitride film 63 as a mask and a partially remaining poly-Si film 64 as a buffer film, selective oxidation is performed. According to this method, the silicon oxide film 61 and the poly-Si film 64 are so combined that the poly-Si film 64 absorbs, scatters and relaxes the stress generated during the selective oxidation. As a result, the silicon oxide films 65 are prevented from being extended to an active area 66 (FIG. 5(c)).
As one current in development of semiconductor integration technology, miniaturization of patterns has been required. As described in connection with FIGS. 4(a) to 4(c), the conventional LOCOS has a problem that it generates the bird's beak to reduce the active area, thereby hindering miniaturization. Thus, a technique of canceling the bird's beak was proposed. In this technique, attention was paid to the fact that when the silicon nitride film is thinner, the silicon oxide films are likely to extend longer to the active area. In order to prevent this extension, the thickness of the silicon nitride film was made thick, or several layers of silicon nitride were laminated. However, in this technique, the entire silicon nitride film becomes thick so that stress is applied to the Si substrate during selective oxidation and disorder in the crystallinity such as crystal defects is generated.
On the other hand, the improved LOCOS method as shown in FIGS. 5(a) to 5(c) can prevent the active area from being reduced even when the silicon nitride film is relatively thin. This method, however, has a problem that it is difficult to control the film thickness by etching in order to make the partial removal of poly-Si film uniformly. In addition, the method has also a problem that a step of forming poly-Si is required as compared with the above LOCOS method so that the number of producing steps increases.